The present invention relates in general to a non-invasive technique for monitoring myometrial activity in pregnant females.
A method of characterizing pregnant female myometrial activity to distinguish between contractions predictive of term or preterm labor and contractures which are not indicative of labor, is disclosed in U.S. Pat. No. 4,967,761 to Nathanielsz, which is hereby incorporated by reference. In this patent, discrete epochs of myometrial contractility are sensed, and signals representing these epochs are produced and stored. Characteristic repetitive frequencies of the signals are determined, which in turn can be used to determine whether the myometrial activity being characterized is predictive of term or preterm labor. The provision of such a technique is invaluable to physicians as an aid in predicting whether or not a pregnant female is going to give birth prematurely. Premature birth accounts for 75% of perinatal mortality and 50% of long-term neurological handicaps, and a major problem with management of preterm birth is the physician's inability to follow precisely the patterns of myometrial activity in pregnant women at risk.
The method disclosed in U.S. Pat. No. 4,967,761 is preferably implemented using either invasive or non-invasive sensors. The sensors can be used to measure electrical impulses, pressure, heat, chemical changes or deformation of the uterine profile caused by the myometrial contractility. The signals generated by the sensors are ultimately stored and analyzed or processed to determine the characteristic repetitive frequencies of the signals, the repetitive frequency being a characteristic of whether the myometrial activity is contractions predictive of term or preterm labor, sometimes referred to as Type I activity, or innocuous contractures, sometimes referred to as Type II activity.
In one invasive technique for obtaining these measurements, myometrial electrodes are implanted in experimental animals to obtain a precise, accurate registration of myometrial contractility continuously throughout pregnancy. Clearly, these methods are too invasive for use in pregnant women. The only known method used to register myometrial activity in pregnant women non-invasively employs a device known as an external tocodynamometer, which is essentially a strain gauge strapped to the abdomen of the pregnant women to register rounding of the uterus as it contracts. It has many disadvantages including relatively poor sensitivity and high susceptibility to movement artifact. It is also ineffective in women more than 160% ideal body weight because subcutaneous adipose tissue interferes with registration of the mechanical effects of myometrial contraction at the abdominal skin surface. This inability to use the external tocodynamometer in overweight women is a major restriction, since many of the women at risk for preterm labor are of this body shape, especially multiparous women. At a recent NIH conference, external tocodynamometry was carefully reviewed by research scientists, obstetricians, industrialists and NIH personnel. The shortcomings of the external tocodynamometer were noted at the conference, and the overall view was that there was a need for a better sensor mechanism. Although large amounts of money are currently being invested by commercial companies in attempts to improve the tocodynamometer, it appears clear that a new type of non-invasive myometrial activity monitoring system is needed.